Semiconductor devices are formed from a flat, thin wafer of a semiconductor material, such as silicon. As the devices and layers of interconnecting circuits are deposited on the wafer, each layer must be polished to achieve a sufficiently flat surface with minimal defects before the next layer can be deposited. A variety of chemical, electrochemical, and chemical mechanical polishing techniques are employed to polish the wafers.
In chemical mechanical polishing (CMP), a polishing pad made of polymer material, such as a polyurethane, may be used in conjunction with a slurry to polish the wafers. The slurry comprises abrasive particles, such as aluminum oxide, cerium oxide, or silica particles, dispersed in an aqueous medium. The abrasive particles generally range in size from 20 to 200 nanometers (nm). Other agents, such as surface acting agents, oxidizing agents, or pH regulators, are typically present in the slurry. The pad may also be textured, such as with grooves or perforations, to aid in the distribution of the slurry across the pad and wafer and removal of the slurry and by products therefrom.
For example, in U.S. Pat. No. 6,656,018, whose teachings are incorporated herein by reference, a pad for polishing a substrate in the presence of a slurry is disclosed, where the slurry may contain abrasive particles and a dispersive agent. The pad itself may include a work surface and a backing surface. The pad may be formed from a two-component system, a first component comprising a soluble component, a second component comprising a polymer matrix component, where the soluble component is distributed throughout at least an upper portion of the working structure and the soluble component may include fibrous materials soluble in the slurry to form a void structure in the work surface.
It is useful to end the CMP process when the desired amount of material has been removed from the surface of the substrate. In some systems, the CMP process is continually monitored throughout in order to determine when the desired amount of material has been removed from the surface of the substrate, without stopping the process. This is typically done by in-situ optical end-point detection. In-situ optical end-point detection involves projecting optical (or some other) light through an aperture or a window in the polishing pad from the platen side so that the optical light is reflected off the polished surface of the substrate and is collected by a detector to monitor the progress of planarization of the wafer surface.